The geometric length between optically effective surfaces of the eye is understood to be included in the optical characteristic variables of an eye of a patient whereat a jump or a non-vanishing gradient of the index of refraction occurs or whereat an absorption of light takes place. The local radii of the surfaces, that is, their Gaussian curvatures can, however, likewise be considered as optical characteristic variables of an eye of a patient. An optical characteristic variable of a patient eye is, inter alia, also the refractive index or the refractive power of an optically effective component thereof.
A surgical microscope of the kind referred to above is disclosed in U.S. Pat. No. 6,004,314. Here, a surgical microscope is described having a coherence interferometer in the form of an OCT measuring unit. This OCT measuring unit provides an OCT scanning beam which can be scanned in a transverse direction across the cornea of a patient eye to determine the thickness thereof and to measure the precise course of the forward surface of the cornea and the rearward surface thereof.
United States patent publication 2003/139736 A1 discloses an ophthalmologic surgical microscope having an integrated refractometer. With the aid of this refractometer, the optical characteristic of an artificial eye lens, which is placed during cataract surgery, can be monitored during the course of the surgical procedure so that it is possible to correct the artificial eye lens as required.
U.S. Pat. No. 5,828,439 discloses a refractometer with which the refractive power of the human eye can be determined during a surgical procedure. A measuring unit is integrated into this refractometer for measuring a non-optical characteristic variable of the eye in the form of the inner pressure of eye. This inner pressure is used to correct a measured refractive power value.
German patent publication 4,310,561 discloses an arrangement for determining refraction and visual acuity during a surgical procedure on the human eye. This arrangement permits imaging a test card on the retina of the eye of the patient which is illuminated by means of a light source and which is suitable for measuring an imaging sharpness of the eye. When the physician detects with the arrangement a sharp test card image on the retina of the patient eye, then an absolute refraction value for the eye being examined can be determined from the apparatus parameters of the arrangement. The device is combined with a surgical microscope and, in turn, permits a physician to make a refraction determination during the course of a surgical procedure when viewing the eye of the patient.
An arrangement for analyzing and correcting the refractive power of a human eye is described in European patent publication 0,247,260. The arrangement includes a data store wherein data of an idealized patient eye are stored for the following: different axial eye lengths; patient age; sex; and, also values for the inner pressure of the eye. In this arrangement, means are provided for determining the particular corneal topography. The arrangement further includes a laser unit with which incremental amounts of the cornea of the human eye can be removed. The removal is so adjusted that a desired topography of an idealized patient eye results. The development of cataract surgery to an ever more careful surgical technique accompanying refractive surgery of the cornea has led to an increase in the demands on the optical characteristic of the patient eye after cataract surgery. It is expected that the full visual acuity is achieved without correcting spectacles.
In addition, in ophthalmologic surgery, it is attempted to correct for reduced visual capacities of the human eye when the pupil width of the eye is greater than 2 mm. These visual deficiencies are caused by the geometric optical correction of the eye which is by nature inadequate. Here it is sought, for patients with average or below average vision, to assist these patients in obtaining an above average visual acuity (that is, >1) via intervention into the optical characteristics of the human eye. This above average visual acuity occurs only in exceptional cases in humans.
Finally, in ophthalmologic surgery, efforts are being made to reverse the reduction of the accommodative power of the human eye because of age which is the so-called presbyopia. Since the effectors of the accommodation are the so-called ciliary muscles, there is a way to improve the accommodative power of the eye in older persons in that the aging natural lens of the eye is exchanged for an artificial lens.
With this as background, it is desirable and necessary for ophthalmologic surgery that the refractive state of a patient eye after surgery can be predicted as precisely as possible, especially the state in the eye interior and the state of the eye lens. For such a prediction, precise optical measurement methods are known and can, for example, be carried out with the IOL Master of Carl Zeiss Meditec AG in the context of a preoperative characterization of the eye.
However, in the present state of cataract surgery, only an intraocular lens is inserted which is selected based on preoperative data. The healing of the wound of the eye is then awaited during which the optical characteristic variables often change such as the distance of the refractive components of the eye. Only after the completion of the healing of the wound do the optical characteristics of the eye stabilize.
In the context of this background, it is desirable to control the optical characteristics of the eye also during surgery to be able to rapidly recognize an unwanted deviation from the preplanned state so that the surgery can be modified as required. Furthermore, there are now surgical techniques (for example, injectionable intraocular lenses) which make it necessary to compensate optical characteristics of the eye during a surgical procedure. Up to now, it is only known in the state of the art to intraoperatively measure a single optical variable of the eye, for example, the refractive power thereof. However, the optical state of the eye cannot be reliably determined on the basis of this single optical characteristic variable. It has been shown that, in the course of a healing process after a surgical procedure on the eye, not only a single optically effective variable changes but a modification of several optical characteristic variables of the eye takes place whose optical effect possibly partially amplifies but also mutually weakens.
The human eye is an optical system which, as a rule, is far from perfect. In a perfect eye, which is also characterized as an emmetropic eye, light rays, which originate from a point in an object region, converge in the eye interior at a point which lies on the retina of the eye. However, in reality, this state never occurs. On the one hand, this is caused by the fact that an optical system, which has a limited aperture, images a mathematical point (whose diameter is zero), because of the diffraction of light, on a point whose diameter is greater than zero. On the other hand, this is caused by the situation that the optical components of the eye, namely cornea and lens, are far from perfect. When the curvature of the cornea is too great or the eye is too long, then the image of the object lies in a plane which is forward of the retina. This leads to the situation that the object is observed by the eye as being blurred. The corresponding vision defect is known as myopia. For the opposite situation, when the cornea is too flat or the eye is too short, the image, which is imaged by the eye, lies behind the surface of the retina. Again, an observed object appears blurred. This vision defect is known as hyperopia. Finally, a third vision defect is called astigmatism and this is caused by the optical areas in the eye having a certain elasticity. This elasticity has the consequence that, with the human eye, it is not at all possible to image a point precisely as a point. Rather, the best possible image of a point that the eye can generate is an ellipse.
Ophthalmologists and opticians know the vision defects listed. They can correct these vision defects with the aid of spectacles and contact lenses. It is also possible to undertake a vision defect correction with a surgical procedure on the eye, for example, by inserting a phakatic intraocular lens in the eye or via the so-called photo-refractive keratomileusis (PRK).
To improve an operative correction of visual defects, it is desirable to detect the condition of the eye during a surgical procedure thereon before, during and after a surgical procedure. Up to now, for example, in cataract surgery, an intraocular lens is inserted into the patient and the form of this lens is determined from preoperatively obtained patient data. In surgery, the optical characteristics of the eye on which surgery is performed cannot be monitored. This is problematic because, during the course of surgery, and during a healing of the wound, optical characteristic variables of the eye can change such as distances of refractive elements.
When it is possible to control the optical characteristics of the eye also during a surgical procedure, an unwanted deviation from a preplanned state can be rapidly detected and adapted or modified during the surgical procedure. In surgical procedures for reestablishing the accommodation while utilizing injectionable intraocular lenses, a compensation of the optical character of intraocular lenses and patient eyes are indeed necessary.